Sport fishermen used to flock to this small city in California's Sierra Nevadas for a chance to catch trophy trout from Lake Davis. These days, silence reigns over the forest-lined lake, and in a few weeks, everything with gills here will be floating upside down.

California officials are set to dump chemicals – including trace amounts of carcinogens – into a future drinking-water supply in a well-publicized effort to protect endangered fish downstream.

The Department of Fish and Game will apply the fish poison in an effort to stamp out the northern pike. Conservationists fear that the invasive predator could spread to the Sacramento River Delta and feast on the delicate populations of native salmon, steelhead, and delta smelt.

A long, open process – including sign-off from the state Department of Health Services – has won over many local leaders and residents, some of whom protested when Fish and Game unsuccessfully treated the waters once before in 1997.

However, a few local activists and other residents say the purported threat of the pike in no way justifies putting chemicals into the lake, a planned source of drinking water for nearby Portola.

Across the US, concerns are rising about invasive species and pesticides, making the relatively common practice of poisoning waterways much more contentious, say experts.

"There is a growing clash between, on the one hand, increasing efforts to save dwindling native fish populations, ... and on the other hand, a growing concern about the potential impacts on the environment and people of pesticides," says Steve Moyer, vice president for government affairs at Trout Unlimited, a conservation group based in Arlington, Va.

The poisoning of Lake Davis 10 years ago touched off more robust resistance nationally to the practice, notes Mr. Moyer. Lake Davis stood out because it had served as a direct drinking- water supply.

At the time, local opposition was fierce. Bill Powers and several others chained themselves to a buoy in the frigid lake. This time around, Mr. Powers – now a county supervisor – supports the poisoning.

"In '97 there were secrets; there were unknowns. The more the local government people like myself asked questions, the more we were stonewalled," says Powers. This time, he says, "every question we asked has been answered."

Fish and Game officials say they have learned from past mistakes. Within two years, pike were spotted in the lake. Working with locals, the agency battled the pike without poison, trying everything from electric shocks to explosives.

But Lake Davis, a man-made lake, suits the pike – a fish that has spread across the West into this northeastern corner of California.

Standing over the lake's dam, two local opponents of the poisoning, Larry Douglas and Dan Wilson, argue that the filters set up below effectively prevent the pike from spreading downstream.

It's very unlikely the pike would get to the delta by its own devices, concedes Ted Foin, a fish expert and consultant on the project at the University of California, Davis. The real danger is that fishermen will move the fish – a crime that now carries penalties of $50,000 or even jail time. The fish is well adapted to the backwaters there, he says, and could be further spread by irrigation systems.

Mr. Douglas and Mr. Wilson say they aren't impressed with that "hypothetical" threat when weighed against what they have learned about the fish poison, CFT Legumine.

"This has never been tested on a human population before. And I have no desire for our children to be the guinea pigs," says Douglas, a former town councilor who carries around a copy of the state's dictionary-sized environmental impact report. The effects of the first treatment can't be adequately studied given Portola's population of more than 2,000.

The actual poison, rotenone, comes from a root, while other chemicals in the mix include benzenes, naphthalene, and methyl pyrrolidone. Government agencies have cited benzene and naphthalene as cancer risks, and methyl pyrrolidone as a reproductive toxin.

However, David Spath with the state Department of Health Services says the bottom line is that the chemical concentrations fall well within legal limits or within their own safety guidelines. All the chemicals are expected to be untraceable within weeks, and until four tests confirm that, the lake will remain closed.

"Our determination is that we don't see any evidence to indicate there would be any short-term or long-term water quality effects on the drinking water in the lake," says Dr. Spath.

That doesn't satisfy Paul Schramski, the state director for the nonprofit group Pesticide Watch.

"Most of these environmental agencies have maybe a few months, a year, to conduct testing for very narrow things that don't take into account the breadth of our concerns," says Mr. Schramski. Specifically, tests often don't consider the interaction between chemicals, long-term risks, or the possibility of chemicals accumulating in animals.

Schramski lauds the action of a Massachusetts health board in 2006, which nixed plans to poison a weed near Natick's Lake Cochituate.

For Wilson, the talk of trace amounts on the order of a few parts per billion still makes him nervous. "It only takes one molecule to infect a cell," he says. "And you might win the lottery tomorrow."

"I would say I'd agree with that statement if we were talking about lifetime exposure conditions," says Jeff Fisher, now with Environ, who led the state's toxicological risk assessment for human health and the environment. "This is a very acute exposure type of scenario, and the public in general is not going to be exposed to it because of the risk management procedures."

Local residents are divided. Several suggested simply draining the lake, an idea considered unfeasible by Fish and Game. Another common response: Get it over with.

"They've got to get rid of the pike," says Margie Dillard, who works at a nearby store. Her husband's fishing guide business has been hurt by the pike domination over the trout population. "Just get it done and over with so we can move on with our business," she says.

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